Hydrocarbon cracking is a term which is well known in the art of petroleum refining and generally refers to the cracking of a large hydrocarbon molecule to a smaller hydrocarbon molecule by breaking at least one carbon to carbon bond. For example, a large paraffin molecule can be cracked into a paraffin and an olefin, and a large olefin molecule can be cracked into two or more smaller olefin molecules. The cracking reaction can also involve the opening of at least one ring of a multi-ring aromatic compound, as well as the cracking of long side chain molecules which may be present on single or multi-ring aromatic compounds.
As known in the art, fluid catalytic cracking is a hydrocarbon cracking process in which vaporized hydrocarbon feed is cracked in the presence of microsperoidal catalyst particles. The catalyst particles are maintained in a fluidized state in the reactor by the vaporized feed, and the fluidized state is enhanced by the addition of steam or a refinery gas stream.
As the catalytic cracking reaction is carried out, the active catalytic cracking catalyst becomes coked (i.e., coated with a carbonaceous material). The activity of the catalytic cracking catalyst decreases as the concentration of the coke deposited on the catalyst increases. Eventually, the catalytic cracking catalyst is deactivated to the point where the catalyst is essentially ineffective in enhancing the equilibrium balance of the cracking reaction under the standard cracking conditions. At this point, the catalytic cracking catalyst is considered to be a deactivated (i.e., spent) cracking catalyst.
Typically, the spent catalyst is regenerated by combusting the coke from the spent catalyst. In order to regenerate the catalyst, oxygen is injected into the regenerator section of the catalytic cracking unit. In the presence of a stoichiometric amount of oxygen, carbon dioxide will be largely present as the carbon combustion product. However, carbon monoxide can also form during the combustion process, and if there is available oxygen, the carbon monoxide can further combust to form carbon dioxide. If this additional combustion occurs in regions of relatively low catalyst density zones, undesirable regions of excess heat can form. This phenomenon is commonly referred to as afterburning. Afterburning is undesirable in that it can lead to a thermal deactivation of the catalyst, an improper regeneration of the spent catalyst, or mechanical damage to the interior of the regenerator vessel.
U.S. Pat. No. 4,849,091 discloses the monitoring of afterburning in a two stage regenerator in order to control dense bed catalyst regeneration. By measuring the temperature of a dense bed catalyst zone, the regeneration gas leaving the dense bed zone, the regeneration gas leaving the riser regeneration zone, or the two regeneration gas streams at the point of initial mixing, and comparing it to the temperature of the combined regeneration gas at a downstream location such as the upper portion of the disengagement space or spent regeneration gas outlet, the resulting differential temperature will indicate the occurrence of afterburning in the upper disengaging zone and, therefore, the presence of oxygen in the disengagement zone. The differential temperature is automatically controlled using a controller to increase or decrease the total amount of oxygen input to the dense catalyst bed.
U.S. Pat. No. 4,354,957 discloses a control system for controlling temperature in the regenerator section of a catalytic cracking unit. The control system uses two input signals to generate one control signal. One of the signals indicates the carbon monoxide content in the overhead gas portion of the regenerator. The other signal indicates the temperature of the gas in the overhead. A computer controller uses these two signals to generate an output signal which controls the total amount of air input into the regenerator.
Although there has been improvement in controlling afterburning as well as controlling the overall temperature within the regenerator section of fluid catalytic cracking units, many problems still exist. Afterburning and overall temperature control could be more effectively controlled, for example, by achieving a controlled distribution of oxygen within the dense phase catalyst bed of the regenerator.